1. Field of The Invention
The present invention relates generally to a valve timing control system for an internal combustion engine. More specifically, the invention relates to a valve timing control system for adjusting opening timing of an intake and/or an exhaust valve of an internal combustion engine in accordance with engine running condition.
2. Description of The Background Art
In recent years, various types of valve timing control systems for adjusting an opening timing of intake and exhaust valves of automotive internal combustion engines have been proposed. For example, U.S. Pat. Nos. 4,231,330 and 4,535,731 disclose a valve timing control system which generally comprises a camshaft disposed in an engine head for controlling the opening and closing of intake and exhaust valves, a cylindrical sleeve for receiving the end portion of the camshaft, and an annular piston arranged between the end portion of the camshaft and the sleeve. The cylindrical sleeve has a sprocket which engages a timing chain so that the sprocket is driven by an engine crankshaft via the timing chain. The sleeve also has an internal toothing which engages an external toothing formed on the annular piston. The annular piston is also formed with an internal toothing which engages an external toothing formed on the end portion of the camshaft. At least one of the two meshing pairs of toothing is helical to form a helical gear. The annular piston is designed to axially slide relative to the camshaft and sleeve by hydraulic pressure supplied from a hydraulic circuit and/or spring force of a compression spring, in accordance with engine running conditions. While the two pairs of toothing remain continuously engaged, the axial sliding movement of the annular piston causes the camshaft to rotate about the sprocket and consequently about the crankshaft which is connected to the sleeve by the chain, so that the opening and closing timings of the intake and exhaust valves can be controlled.
As mentioned above, in the aforementioned conventional valve timing control systems, the relative rotation of the camshaft to the sprocket is caused by the helical toothing formed on at least one of the two meshing pairs of toothing. Such a helical toothing must be very accurately processed in order to assure high accuracy of meshing of the toothing. As a result, manufacturing of the toothing becomes complicated, labor effectiveness of processing decreases and the cost thereof increases.
Furthermore, in the aforementioned conventional valve timing control systems, the helical gear extends along the axis of the camshaft and is designed to move along same, furthermore, a large space is required for receiving the valve timing control system. As a result, the size of the internal combustion engine becomes large.